The invention described in this document concerns a device that is placed in a fishing trawl that allows the escape of unwanted species during trawling. The device is placed in the trawl just ahead of the trawl bag.
A fishing trawl is basically a large net that is towed by a fishing vessel, along the bottom of the ocean and captures any organism that is unable to get out of its path. The trawl has a large opening on one end and, like a large funnel, narrows to a bag on the other end where the captured animals accumulate. Fishing trawls are typically very non-specific in the capture of animals, that is, they capture many un-wanted species. These unwanted species are referred to as “bycatch” or in some cases “bykill”. Estimates range from 7.3 to 39.5 million tons of bycatch discarded into the oceans world wide. Shrimp and bottom fish trawls have been estimated to account for over 50% of the total. In the U.S. Gulf of Mexico shrimp fishery, the ratio of bycatch to shrimp is 4.56, that is, 4.56 pounds of fish are captured and discarded for every pound of shrimp.
The vast majority of bycatch species are discarded either dead or dying. Estimates suggest that 90+% of bycatch species are dead when discarded. The effect of the introduction of large numbers of dead and dying organisms into the marine environment is unknown but likely detrimental to the ecosystem.
Commercial fishing is an important source of revenue for many coastal communities and provides an important source of protein in many parts of the world. Considering that many fisheries are on the decline worldwide, the need for sustainable fishing practices is more important today than ever before. Sustainable fishing must include fishing techniques that are more efficient at capturing the target species. In some cases, commercial fishing activities are limited based upon the capture of the targeted species (quotas) and in some cases may be based on bycatch of non-target species. In some U.S. fisheries, commercial fishing is not possible or profitable during certain times of the year or in certain areas because of the presence of large numbers of bycatch species. Large amounts of bycatch require the fisher to spend excess time sorting target from non-target species and limits the time spent in actual fishing. A device that reduces bycatch during trawl fishing is good for the environment, good for the commercial fisher, and good for the economy. In a world of declining fish stocks, it is imperative that fishers use every weapon in their arsenal to reduce or eliminate the catch of unwanted species.
A variety of devices have been developed for reducing bycatch of species during trawl fishing. Several devices have been proposed for the reduction of sea turtle bycatch during shrimp trawling. U.S. Pat. No. 4,739,574, from 1988 describes a device that excludes sea turtle bycatch during trawling. The device consists of a grid with sufficient spacing to allow small species (such as shrimp) to pass into the trawl bag but large species such as sea turtles, to be directed to an escape opening. The turtle excluder device is placed in the trawl body in front of the trawl bag or cod end. U.S. Pat. No. 5,076,000 from 1991 describes a weedless turtle excluder device. Additionally, U.S. Pat. No. 5,222,318 from June 1993 describes a turtle exclusion grate having bowed vertical bars and an overhang at its base. An apparatus for exclusion of debris and bycatch from bottom trawls is described in U.S. Pat. No. 5,575,102 from November 1996. That patent describes a grid-type turtle and debris excluder device employing a spoiler plate that directs the flow of water into the excluder grid and creates a water curtain.
In addition to turtle excluder devices, various inventions have been designed for use in shrimp and bottom trawls that are purported to separate targeted species from unwanted species. For example, U.S. Pat. No. 3,195,261 from 1965 describes a spiller device for trawl nets that allows fish to escape when the trawl bag becomes overloaded. U.S. Pat. No. 3,440,752 from April 1969 regards a bycatch reduction device that employees a grill positioned within the trawl bag that separates marine species by size. A pump and a pumping tube allow the desired species to be pumped to the fishing vessel. U.S. Pat. No. 4,805,335 from February 1989 describes a sorting device for trawls that assumes a funnel configuration at the bottom of the trawl and allows shellfish to escape while retaining fin-fish. An apparatus for exclusion of debris from bottom trawls, described in U.S. Pat. No. 4,869,010 from September 1989, is an array of angled, spaced bars (a grid) placed immediately ahead of the trawl bag. The grid allows shrimp to pass while fish are forced to the outside of the trawl. U.S. Pat. No. 5,123,195 from June 1992 describes a system to separate marine shrimp from debris and unwanted species. The system employs a separate net, piggy-backed upon the main net. A sonic pulser is used to stimulate shrimp to enter the upper net while debris and unwanted fish pass out of the back of the trawl. U.S. Pat. No. 5,215,197 from June 1993 sorts trawl captured marine species on the fishing vessel using several sorting screens arranged vertically such that the screens, from top to bottom, have progressively smaller openings. The largest organisms are retained on the first screen while smaller ones pass through. U.S. Pat. No. 5,325,619 from July 1994 describes a fishing trawl device that uses an outer net of a certain mesh size and an inner net of smaller mesh size to sort species and debris by size. U.S. Pat. No. 6,223,462, from 2001 describes a device that is a venturi-type narrowing of the trawl just ahead of the trawl bag. The device has an outslip unit with a number of grid frames attached to each other that allows escape of fish below a certain size but retains fish too large to pass through the grid.
In most of the above described devices, separation of target from non-target species is brought about by size sorting. Grids or screens are used to retain large species while smaller one are not retained. This may be effective for reducing bycatch when target and non-target species are of significantly different sizes. However, in many (most?) shrimp trawl fisheries, the target species (shrimp) are not significantly different in size from bycatch (small fish) such that size sorting is not effective for reducing bycatch. The device described herein takes advantage of fundamental differences between shrimp (target species) and fish (non-target species) behavior and uses these differences to separate them during trawling.